This invention relates to a process for preparing compatible thermoplastic polymer blends from two or more normally incompatible polymers. More particularly, this invention relates to blends of a first polymer containing pendant cyclic iminoether groups and another polymer containing coreactive groups which react with cyclic iminoether groups to form linkages between the polymers.
The reaction of cyclic iminoether ether groups, such as a 2-oxazoline ring, with a coreactive group, such as a free carboxyl or anhydride, is well known in the art. Such examples are disclosed in U.S. Pat. Nos. 4,508,869; 4,474,923; 4,247,671; 4,245,063; 3,535,291; and 3,509.235 and Japanese Publication KOHO No. 44-30384. While the basic ringopening reaction of 2-oxazoline is understood, it is still desirable to find novel ways to utilize this reaction.
One attempt to employ the crosslinking characteristic of 2-oxazoline with a carboxyl group is disclosed in U.S. Patent No. 3,509,235 to Riemhofer. Riemhofer discloses the preparation of thermosetting compositions having good film characteristics by employing the 2-oxazoline to obtain the desired degree of crosslinking. The crosslinking reaction takes place during the curing of the polymer mixture which is heated from 100.degree. to 200.degree. C.
Unlike Riemhofer, the subject polymer mixture is formed into a thermoplastic blend which can be reheated and molded over and over again. This is not possible with a thermoset which forms an irreversible solid upon curing. An additional advantage of the subject invention is that novel thermoplastic blends are prepared from normally incompatible polymers which heretobefore was not possible. That is, normally incompatible polymers are compatibilized by treating them such that they respectively contain 2-oxazoline groups and coreactive groups such that covalent bonds can be formed between them sufficient to compatibilize the incompatible polymers into a thermoplastic blend but insufficient to form undesirable amounts of crosslinking.
Often, it is desirable to blend polymers to take advantage of the individual component polymer characteristics. It is normally expected that polymer blends will have properties which are approximately the average of those of the component polymers. In other words, it is expected that blends of compatible polymers will exhibit mechanical properties, ESCR, moldability, thermoformability, and like properties which are intermediate to those of the component polymers.
In some situations, it is desirable to prepare polymer blends from component polymers which are incompatible. Unfortunately, polymer blends prepared from incompatible polymers exhibit poorer properties than the component polymers.
For example, monovinylidene aromatic polymers such as polystyrene are known to be incompatible with polyolefin polymers which would otherwise be advantageously blended therewith, Thus, most blends containing polystyrene and polyolefin polymers exhibit poorer properties than either of the component polymers.
Various methods have been proposed to prepare blends of normally incompatible polymers. Generally, these methods have focused on the use of grafting techniques or the use of a third component, a compatiblizing agent, in the blend. For example, in U.S. Pat. Nos. 4,386,187 and 4,386,188 it is taught to prepare blends of polyolefins and a polystyrene using a styrene/butadiene/styrene block copolymer. Compatibilizing agents which are ethylene/vinylacetate copolymers, ethylene/acrylic acid ester copolymers and ethylene/methacrylic acid ester copolymers have also been taught for use in preparing blends of polystyrene and polyolefins (see Japanesse Patent Announcement Kokai No. 48-43031/1973). Other such compatibilizing agents are taught in, for example, U.S. Pat. Nos. 4,188,432; 4,020,025; British Patent No. 1,363,463 and German Patent No. 241,375.
Unfortunately, these approaches to preparing compatible polymer blends often do not yield entirely satisfactory results. In many instances, the type and proportion of the component polymers which can be blended using these techniques is quite narrowly restricted. In addition, the inclusion of an additional component in the blend often has an adverse effect on the properties of the blend. Moreover, the blend achieved is still not as compatible as desired and, accordingly, the properties of the blend are generally not as good as expected.
Therefore, it is desirable to provide a blend of normally incompatible polymers in which improved compatibilization of the polymers and improved properties of the blend are achieved.